This invention relates to an apparatus such as weigher, wind-tunnel balance, pressure gauge or the like which is used for measuring a force or mass as a digital value and, particularly, to a device for compensating for a time-dependent error caused by creep of a flexural member of a force detector used in such apparatus.
For example, a load detector included in a weigher is adapted to convert deflection or strain caused by a load into an electric signal, using a lever or Roberval mechanism or such mechanism co-operating with springs (hereinunder referred to as "first type"), or a flexural member whose strain is electrically detected by a strain gauge (hereinunder referred to as "second type"). While such detectors generally produce an output signal as soon as the load is applied thereto, the signal value may vary slightly with a lapse of time to result in erroneous indication. This effect is referred to as "creep" and a technique for compensating for such creep error is disclosed, for example, in U.S. Pat. Nos. 4,412,298 and 4,691,290.
However, it is necessary to apply temperature correction to the amount of compensation in the abovementioned first type of detector in order to improve its indication accuracy, since its creep characteristic often varies with its surrounding tempearture. On the other hand, in the second type of detector such as strain gauge load cell including mechano-electric convertors attached or adhered to a flexural member, a delicate difference may appear during a lapse of time between the strains in both flexural member and convertor. As will be described in detail later, this is due to "stress relaxation". In such type of detector, therefore, it is necessary to effect overall compensation for both of the well-known creep error and the abovementioned stress relaxation error, as well as temperature correction of the amount of compensation as in the first type of detector. However, the devices of the above-cited prior art are believed to be incomplete in compensation for such time-dependent indication errors, since there is neither description nor suggestion about the abovementioned problem in the above-cited references.
Accordingly, a first object of this invention is to provide an improved compensation device which can apply temperature correction to the amount of compensation for the time-dependent indication errors due to creep of the detector.
A second object of this invention is to provide an improved compensation device which can apply correction for stress relaxation error to the amount of compensation for the time-dependent indication errors due to creep of the detector.
A third object of this invention is to provide an improved compensation device which can apply temperature correction to the amount of compensation for the time-dependent errors due to creep and stress relaxation of the detector as occasion demands.